A surgeon may use a variety of surgical instruments when performing a hip arthroplasty to implant a prosthesis such as an acetabular cup into a patient's acetabulum (or otherwise, when performing other orthopaedic surgeries to implant a prosthesis into a patient's anatomy). For example, the surgeon may use a reamer (or other cutting device) to mill out the acetabulum and thus form a socket within which an artificial cup can be anchored. An impactor may also be used to drive the cup into place within the acetabulum.
When operating, the surgeon must take care that the instruments are oriented as precisely as possible, so that the acetabular cup will ultimately be positioned and oriented as intended with the greatest possible precision. Otherwise, if the acetabular cup is not properly positioned (for example, if it has too shallow or too high of a cup inclination angle), the patient may experience excessive wear on the acetabular cup or components used with the acetabular cup. Other problems can include dislocation, impingement, limited ranges of motion, infection, or rejection of the implant. In addition, improper alignment may be more likely to occur if the surgeon performs the surgery freehanded.
Additionally, the surgeon must take care not to over-drill or over-ream the acetabulum. For example, in some surgical techniques, guide rods are inserted within the acetabulum to guide other surgical instruments (such as a reamer or an impactor) during the surgery. If the guide rod is inserted too deeply within the acetabulum, the guide rod can puncture or otherwise compromise the strength of the medial wall of the acetabulum, or injure other anatomy behind the acetabulum. Similarly, surgical instruments such as a reamer may ream too much (or too little) of the bone within the acetabulum.